1. Field of the Invention
The present invention relates to a wireless communication apparatus which receives a radio signal transmitted from a transmitter and a wireless communication method. The present invention relates more particularly to a wireless communication apparatus which receives a radio signal encoded and digitally modulated by a transmitter and a wireless communication method.
More specifically, the present invention relates to a wireless communication apparatus which performs reception processing while performing symbol tracking so as not to lose synchronized symbol timing obtained under conditions in which a frequency shift exists between an oscillator mounted in a transmitter and that mounted in a receiver, and to a wireless communication method. In particular, the present invention relates to a wireless communication apparatus which is applicable to a constant envelope modulation system and performs symbol tracking without substantially increasing any burden to an analog-to-digital (A/D) converter in order to accelerate a symbol rate, and a wireless communication method thereof.
2. Description of the Related Art
Wireless communication encompasses a wide range of roles from high-capacity trunks such as terrestrial broadcasting, terrestrial microwave communication, satellite communication, or satellite broadcasting to access lines such as mobile communication lines. Recently, digital wireless communication, which involves transmission of digital data via radio waves, such as digital broadcasting or a wireless local area network (LAN) has become popular.
In digital wireless communication, source and channel coding and digital modulation are performed on a transmission signal by a transmitter. Digital demodulation and source and channel decoding, which are the reverse of the processing performed by the transmitter, are performed by a receiver. According to digital communication techniques, high-speed and high-capacity communication can be realized. Moreover, noise immunity, interference immunity, and distortion immunity increase, and thereby high-quality communication can be realized.
In digital wireless communication, for example, a spread spectrum (SS) system can be used. That is, a digital transmission signal which is spread using a signal called a spread code to have a band wider than the original digital signal is transmitted from the transmitter. The transmitted signal is decoded into the original digital signal by using the same spread code at the receiver. According to an SS system, a required C/I level for realizing normal communication can be set to be lower than 0 dB even in an environment where communication systems with the same frequency band exist. Such an SS system is used in wireless LANs with IEEE 802.11 series, Bluetooth communication, CDMA cellular telephones, and the like. SS systems include a direct-sequence spread spectrum (DSSS) system in which an occupied band is spread by multiplying an information signal at a transmitter by a series of random codes, referred to as a pseudo-noise (PN) code, and a frequency hopping spread spectrum (FHSS) system in which a carrier is switched rapidly among many frequency channels.
In wireless communication, a transmitter and a receiver each have a local oscillator mounted therein. There exists a slight error, namely, a frequency offset, between frequencies of such local oscillators mounted in the transmitter and the receiver. For example, in the case of a wireless LAN, oscillators accurate to about 20 ppm are employed. Such an error between the local oscillators located in analog sections of the transmitter and the receiver is observed as a phenomenon in which reception sampling points shift with the passage of time at the receiver. The shift of the reception sampling points not only causes a signal-to-noise (SN) ratio of the reception sampling points to decrease but also causes a serious problem that the reception data is not demodulated into the original data if the amount of the shift becomes more than one symbol period.
FIG. 9 shows an eye pattern of a reception signal. An eye pattern is a diagram showing parts of a waveform of a signal extracted in predetermined time intervals, which are superposed in the same region. Even in the case where sampling is performed at a point A with a high SN ratio in FIG. 9, if a frequency offset exists between the transmitter and the receiver, a sampling point shifts from the point A to a point B and the SN ratio of the sampling point decreases.
In order to solve such a problem, it is necessary that reception processing be performed by the receiver in consideration of the frequency error between the transmitter and the receiver. One of such reception method is to provide a scheme in which symbol timing is regenerated so as to follow the frequency offset between the transmitter and the receiver and not to lose synchronized symbol timing obtained in the communication system. Such a scheme is referred to as “symbol tracking”.
FIG. 10 shows a functional block diagram of a symbol tracking device. A shown example employs a tracking system called a delay locked loop, which is generally used in a DSSS system (see, FIG. 12 of Japanese Unexamined Patent Application Publication No. 2003-32225; and Mitsuo Yokoyama, “Supekutoru-Kakusan Tsushin Shisutemu (Spread Spectrum Communication System),” Kagaku-Gijutsu Shuppansha, 1988). In the tracking system, an error signal is generated by obtaining a difference between integrated amplitude values of signals which are sampled at points Tc/2 before and after a target sampling point, which are Early and Late points, respectively, where Tc represents a symbol period, and thereby symbol tracking is performed. Note that the shown example assumes a DSSS system; however, the shown example may also be used in communication systems besides SS systems.
Such a symbol tracking system employs an amplitude value as tracking information. Thus, if a constant envelope modulation system in which an amplitude of a high-frequency signal does not temporally change, such as π/2 shift binary phase-shift keying (BPSK), is used, there is a problem that it is difficult to obtain a difference between the integrated amplitude values at the Early and Late points. In addition, generally, X-times (X being natural number 4 or higher) oversampling is performed on each symbol, and a processing load on an A/D converter increases in accordance with an increased symbol rate.